The use of radioactive gas as a means to generate light is well known in the art. Typically, radio-luminescent light sources include the combination of a phosphor and a radioactive gas such as tritium enclosed within a sealed cylindrical, spherical or rectangular chamber. Tritium, a hydrogen molecule with one proton and two neutrons, is a radioactive beta emitter having a half life of about 12.3 years. One example of a radio-luminescent tritium light source is tritium-filled containers found on 747 jets for indicating the direction of exits in the case of catastrophic power failure.
Another example of a tritium light source is disclosed in U.S. Pat. No. 4,677,008 to Webb, issued Jun. 30, 1987. In this patent, tritium and at least one phosphor particle are disposed within a gas tight envelope. These self-luminous microspheres are disclosed for use on surfaces to form signs, markers, indicators and the like. In addition, a plurality of the self-luminous microspheres may be disposed in a transparent binder to form a luminous paint.
The self-luminous microspheres typically have an output, measured in foot-Lamberts as a unit of flux per unit source area, between about 1 foot-Lambert and 10 foot-Lambert.
These self-luminous microspheres also provide advantages over other prior art designs, including safety and efficiency.
It is also been proposed to use radio-luminescent light sources in conjunction with the generation of power. Direct conversion devices have been proposed wherein a semiconductor material used as a photovoltaic convertor is placed adjacent the radioactive source. Electrons from the radioactive source strike the lattice of the semiconductor imparting energy which frees electron and hole pairs. The electrons/hole pairs then create a bias voltage which can be tapped for current. Drawbacks associated with these direct conversion devices include damage to the lattice of the semiconductor as a result of impact by the high energy particles emitted from the radioactive source. In addition, when using tritium as the radioactive source, hydrogen can passivate the semiconductor material resulting in still lower efficiencies.
Indirect conversion power sources have also been proposed. In these devices, the radiation from the radioactive source first strikes a phosphor which then releases a photon of light. If the energy of the released photon of light is in the bandgap absorption wavelength, it is accepted by an adjacent photovoltaic cell and converted into an electron/hole pair with a certain energy. Efficiencies for these types of devices are generally about 10%.
U.S. Pat. No. 5,082,505 to Cota et al discloses a self-sustaining power module of the indirect conversion type. In this patent, the radioactive source is a tritium-containing capsule that interfaces with the receptor surfaces of a photovoltaic cell. The capsule has inside surfaces that are coated with phosphor and also contains the tritium gas. The tritium gas produces beta particles that bombard the phosphor causing the release of the photons. The photons, in turn, strike and cause the photovoltaic cell to generate a current flow that is then applied, via a pair of electrodes, to an external load. These devices can come in a plurality of modules to provide various output combinations.
Drawbacks associated with the prior art indirect conversion power sources include a limited area of phosphor for use since the phosphor is coated on the inside surface of the tritium-containing container. Since only one surface of the phosphor is available for photon generation, less light is produced. In addition, any generated light must diffuse through the overall phosphor coating to escape the enclosing vessel.
Furthermore, these prior art types of conversion devices also permit beta particle absorption by the tritium gas itself. Since the beta particles emitted by the tritium are weak and have a travel range of only about 5 to 30 microns, the beta particles can be absorbed by the tritium gas before striking the phosphor. At low pressures, beta particle absorption is not as prevalent since the density of the tritium gas molecules is low enough that few beta particles are absorbed. However, when using high gas pressures, the gas density increases, thereby also increasing the likelihood of absorption of the beta particles by the tritium gas.
In view of the disadvantages mentioned above, a need has developed to provide an improved power source which overcomes the deficiencies in the prior art discussed above.
In response to this need, the present invention provides a novel power source using self-luminous microspheres in combination with a photovoltaic source to provide improved energy efficiency and light output. The present invention also provides a flexible and compact form which is readily adaptable to different power requirements and shape configurations.